In a laser recording device that forms an image according to image signals by scanning a recording medium with laser beams from laser diodes, light intensity control is performed to regulate the laser output of the laser diodes.
When a threshold current Ith is a current at which a laser diode suddenly starts to emit light and a light-emission current Iη is a current that is greater than the threshold current Ith and required to obtain a desired laser output, the threshold current Ith and the light-emission current Iη vary depending on the laser diode because of variations in manufacturing processes. Still, the light intensity of a laser diode linearly increases as the light-emission current Iη increases. In other words, the light-emission current and the light intensity of a laser diode are proportional.
Normally, in a laser diode driving device, an operating current Iop (bias current Ibi+switch current Isw), which consists of a bias current Ibi to be constantly supplied to the laser diode and a switch current Isw to be supplied according to a modulation signal based on image information, is controlled based on detected actual laser outputs to stably obtain a desired laser output.
In light intensity control in such a laser diode driving device, the response characteristics of a laser diode to an input pulse signal during image formation can be improved by making the bias current Ibi smaller than and as close as possible to the threshold current Ith. Therefore, the bias current Ibi is normally set at a value slightly smaller than the threshold current Ith.
There are several known technologies for detecting the threshold current Ith for each laser diode and setting the bias current Ibi based on the detected threshold current Ith.
For example, patent document 1 discloses a technology for detecting the threshold current Ith using multiple reference voltages and a sample-and-hold circuit and setting the bias current Ibi based on the detected threshold current Ith. However, with the disclosed technology using a sample-and-hold circuit that holds an analog voltage in a capacitor, the bias current Ibi may fluctuate because of a leakage current during a holding period. Particularly, when the holding period becomes long, the fluctuation caused by the leakage current becomes larger.
As another example, patent document 2 discloses a technology where the light intensity of a laser diode is detected with an A/D converter and the bias current Ibi and the switch current Isw are set by D/A converters. Using D/A converters to set the bias current Ibi and the switch current Isw makes it possible to eliminate the above mentioned problem caused by a leakage current in a sample-and-hold circuit. However, although it depends on the setting accuracy of light intensity, D/A converters with a resolution of 10 bits or greater are necessary for the above purpose. Such high-resolution D/A converters increase the chip size and costs of a semiconductor integrated circuit used for a laser diode driving device.
Meanwhile, in the field of laser printers and digital copiers that use laser diodes as light sources, there is a growing demand for high-resolution and high-speed devices. To increase the resolution and printing speed of a device using only one laser beam, it is necessary to increase the modulating speed that is a speed of turning on and off a laser diode according to input image data. However, there is a limit to this approach of increasing the modulating speed. Therefore, to further increase the resolution and printing speed of a device, it is necessary to increase the number of laser beams. Assuming that the modulating speed and the printing speed are the same as those when one laser beam is used, using four laser beams makes it possible to double the resolution in the main- and sub-scanning directions (lengthwise and crosswise). When the resolution is the same, using four laser beams makes it possible to increase the printing speed by four times.
Semiconductor lasers used as light sources are roughly divided into two types: an edge-emitting laser that emits a laser beam in a direction parallel to the active layer and a surface-emitting laser that emits a laser beam in a direction perpendicular to the active layer. Edge-emitting lasers that emit one, two, or four laser beams are popularly used for printers and copiers. When eight laser beams are necessary, for example, eight single-beam lasers each emitting one laser beam, four multi-beam lasers each emitting two laser beams, or two multi-beam lasers each emitting four laser beams may be used. The price of a laser increases as the number of emitting beams increases. The positional relationships between optical axes of laser beams of a multi-beam laser are stable. On the other hand, when single-beam lasers are used to provide multiple laser beams, it is necessary to adjust the positional relationships between optical axes of the laser beams by adjusting the positions of the single-beam lasers in an apparatus. Therefore, using a multi-beam laser makes it easier to adjust positional relationships between optical axes of laser beams.
Also, because of structural limitations, it is technically difficult to increase the number of laser beams of an edge-emitting laser. Meanwhile, the number of laser beams of a surface-emitting laser can be comparatively easily increased. Therefore, a surface-emitting laser is preferable to increase the speed and resolution of a device by increasing the number of laser beams. For the above reasons, developments of devices such as laser printers and digital copiers using surface-emitting lasers capable of emitting a large number of laser beams as light sources have been very active these past years to meet the demand for high-speed, high-resolution devices. For example, a surface-emitting laser may be configured to emit 30 to 40 laser beams.
Generally, an edge-emitting laser includes an internal light-receiving element. Therefore, for example, when two multi-beam lasers each including one light-receiving element and emitting four laser beams are used to provide eight laser beams, one fourth of a light-intensity adjustment period can be used for adjusting the intensity of each laser beam. Meanwhile, a surface-emitting laser, because of its structure, cannot include an internal light-receiving element and therefore has to use an external light-receiving element. Also, because the axes of laser beams are close to each other in a surface-emitting laser, it is difficult to separately detect multiple laser beams with different light-receiving elements at once. For these reasons, for example, when a surface-emitting laser is capable of emitting 40 laser beams, only one fortieth of a light-intensity adjustment period can be used for adjusting the intensity of each laser beam.
During one scanning period (for scanning one line), time usable for adjusting light intensity is very limited. For example, when a light-intensity control device capable of adjusting the intensities of four laser beams during one scanning period is used to adjust 40 laser beams of a surface-emitting laser, the intensities of the 40 laser beams can be adjusted only once during 10 scanning periods. This is particularly a problem when a sample-and-hold circuit is used to generate a bias current. With a sample-and-hold circuit, a detected sample value must be held in a capacitor while the light intensity of the corresponding laser beam cannot be adjusted. For example, the holding period required when the light intensity can be adjusted only once during 10 scanning periods is ten times longer than that required when the intensity adjustment can be performed once in each scanning period. In such a case, the fluctuation in the generated bias current caused by a leakage current becomes too large to be ignored.
FIG. 13 is a schematic block diagram illustrating an exemplary circuit configuration of a related art laser diode driving device. Below, the number of bits required for a D/A converter for setting a light-emission current is described with reference to FIG. 13.
In FIG. 13, a bias current control circuit controls a bias current that is smaller than a threshold current and a switch current control circuit controls a switch current that is proportional to the light intensity. Also, it is assumed that the light intensity is controlled within a setting range and the light-emission current is adjusted within a setting range between 1.25 mA and 5 mA. For example, to set the light intensity in units of 0.5% or smaller (with a resolution of 0.5% or smaller), it is necessary to be able to set the light-emission current in units of 0.5% or smaller (with a resolution of 0.5% or smaller).
Based on the above assumptions, when the maximum output current of a D/A converter is 5 mA, a setting scale of at least 200 steps (a resolution of 0.5% or 6.25 μA) is necessary to output the minimum output current of 1.25 mA. When a setting scale has 250 steps (a resolution of 0.4% or 5 μA) per 1.25 mA, 1000 steps (5 mA/5 μA) are necessary per 5 mA. Thus, in this case, a D/A converter with a resolution of 10 bits is necessary to set a light-emission current.
Meanwhile, as the number of bits of a D/A converter increases, higher output accuracy is required. Also, when the number of bits of a D/A converter increases by one, the area of the D/A converter on a semiconductor integrated circuit (IC) becomes two times larger. For example, to drive a multi-beam laser capable of emitting 40 beams, it is necessary to provide 40 D/A converters with a 10-bit resolution on the same IC. Accordingly, such a configuration increases the chip size and costs of a semiconductor integrated circuit.
[Patent document 1] Japanese Patent No. 3687621
[Patent document 2] Japanese Patent No. 3365094
As described above, there is a trend to increase the number of laser beams to improve the speed and resolution of a device. However, increasing the number of beams increases the period of time during which the light intensity of a laser beam cannot be adjusted. Therefore, if an analog sample-and-hold circuit is used to adjust light intensity, the bias current may fluctuate because of a leakage current during a holding period. Meanwhile, with a method using D/A converters, which generate predetermined currents based on a constant current and digital data, the above problem can be prevented. However, this method requires a laser beam driving circuit, which is equivalent to that used to drive a single-beam laser, for each of the laser beams of a multi-beam laser and therefore increases the chip size and costs of a semiconductor integrated circuit used for a laser diode driving device.